1. Field of the Invention
This invention relates to the field of video monitoring, wherein television is used to view a scene and to actuate an alarm if movement or change takes place in the light distribution emanating from the viewed scene.
2. Description of the Prior Art
Video systems having the capability of detecting motion or other change in a viewed scene have been known to have applications in security against intrusion of a viewed area, and in safety monitoring and control of industrial processes. Such systems compare information derived in a television image frame with analogous information derived during a previous frame, and trigger an alarm if the detected difference is greater than a predetermined threshhold. Such systems have included both analog and digital apparatus and circuitry.
Analog systems have been proposed which utilize the main characteristic of the video signal, which is an amplitude modulated signal, and process it in one or more of a variety of known fashions to derive the desired information from frame to frame.
While analog systems are generally characterized as being relatively fast operating, simple and inexpensive, they have been regarded as lacking flexibility and accuracy.
Partially in order to overcome these difficulties, digital systems have been proposed. Such systems typically include a television system and equipment for digitizing image frame information derived by the television. Such systems often require interfacing to a digital data processing system, such as a digital computer, for utilizing the information developed by the television system to detect changes from frame to frame.
Digital systems have generally been regarded as too complex and expensive for simple security alarm applications, and for performing simple industrial monitoring functions. The necessity to interface with a computer to compare light distribution patterns from among different image frames or to represent a sufficiently accurate image, results in digital systems being excessively costly, often well in excess of $5,000 per system.
The complexity of interfacing an analog television system with digital equipment gives rise to a relatively high degree of difficulty in maintaining effective and reliable operation of the large number of system components which must cooperate to be effective.
In one proposal for a digital system, the system compares fixed points during each video scan, storing up information about the observed points. During subsequent scans, the system compares information derived from the newly obtained points. This complex system first digitizes the video signal before sampling. Each sample is stored in a digital memory system indicating its "X" and "Y" coordinates corresponding to sample point location.
Such a system samples more than 16,000 points in each image frame. The amplitude of the video signal at each point is digitized and stored with its coordinate location information, which is also in digital form. The amplitude value of each point of a subsequent frame, after being stored, is subtracted from corresponding values in a previous frame, and the difference information for each point is also stored. Only then can the digital data processing system be used to develop information relating to image aspects such as the existence, size and speed of an intruding or moving object or person. Time and magnitude of the intrusion, along with the locations of the objects which caused it, are recorded in digital form.
According to another proposal, in another digital system, an entire image frame is scanned, and a counter counts the number of times the video level exceeds a predetermined threshhold. Such a system does not incorporate an organized sampling system as in the previously described proposal. One feature of the image could be completely overlooked because the system rejects all of the image components as to which the video signal is below the threshhold. This system suffers from the disadvantages of the previously described proposal as well, such as high expense and complexity.
The disadvantages of the prior art systems and proposals are obviated or ameliorated by the subsequently described video monitoring system which embodies the present invention. It is a general object of this invention to provide a video monitoring system which incorporates the accuracy and flexibility of a digital system, while partaking of the speed, simplicity and relatively low cost of analog systems.